The present invention is in the field of time delay devices, such as those that may be used for the control of phased-array radars, communication systems, or correlators.
This invention relates to apparatus for producing true-time delay devices, such as those useful in the control of phased array radars. It is desirable to use a system that produces signals to control the timing of the emission of each of a plurality of electromagnetic radiation beams, delaying each of them in time by some time increment. The delay in each signal should be capable of being controlled independently of the other signals.
Phased array radars have the advantage that the radar beams can be steered electronically by changing the phase or timing of the signal radiated by the individual elements of the array. Often, this is accomplished by controlling the phase of the signals applied to the array elements. This procedure introduces undesirable squint if very short pulses or broad bandwidths are required. True time delay offers a scheme for controlling the elements without squint even with broadband signals.
Electronically implementing the true time delays is generally impractical because of the need for many long lengths of strip line, waveguides, or coaxial cable, which are expensive, bulky, and temperature sensitive. Because long paths are comparatively easy to obtain optically, photonic systems present a means of obtaining the beam agility of array systems combined with wide bandwidth. Approaches to true time delay tend to fall into two categories: those using fibers and those using long free-space paths. Some fiber approaches use multiple optical switches or broadcast the light over all possible paths at once. Wavelength-division-multiplexing schemes have recently been developed by use of fiber Bragg gratings. Free-space systems have also used multiple optical switches for switching the beams between sequential optical paths. These optical switches are usually liquid-crystal based.
It is therefore an object of the current invention to create a device for optically generating true time delays that is inexpensive, is compact in design, and is sufficiently temperature insensitive.
Although described with respect to the field of phased-array radars, it will be appreciated that advantages similar to those obtained in optically producing true-time delays, as well as other advantages, may be obtained in other applications of the present invention requiring timing functions. These advantages may become apparent to one of ordinary skill in the art in light of the present disclosure or through practice of the invention.
The present invention includes time delay devices and time delay systems. The invention also includes machines and instruments using those aspects of the invention. The invention may also be used to upgrade, repair, or retrofit existing machines or instruments, using methods and components known in the art.
The present invention includes a true time device that falls into the free-space category but uses a multiple-pass optical cell with refocusing mirrors that has the advantage of avoiding beam-spreading problems. This approach differs from previous free-space approaches in that it uses only one optical switch or spatial light modulator instead of one or more switches for each bit. In this approach, the microwave signal for each antenna element may be modulated onto an optical beam. After the individual optical beams are delayed by the desired amount of time, the signals may then be down-converted to microwave signals for further processing. This process may be used in either the transmit or the receive mode of the phased array radar.
An apparatus of the present invention for optically generating time delays in signals uses an input light source to generate at least one individual light beam from at least one direction. An input mirror reflects the light beam(s) to a plurality of optical elements configured so as to define a plurality of possible light paths for each light beam. Here, the optical elements comprise two delay engines. Preferably, at least one refocusing optical element may be used used to restrict the divergence of a light beam diverted through a light path. A path selector, such as a three-state spatial light modulator then selects a path from among these light paths for each pass of a light beam through the optical elements. An output mirror then reflects each light beam emerging from the optical elements to at least one receiving device. The receiving device receives a light beam reflected by the output mirror and extracts the delayed signal from the light beam.
The present invention also includes an apparatus for optically generating time delays in signals utilizing a roof prism. An input light source generates at least one individual light beam from at least one direction. An input mirror reflects the light beam(s) to a flat mirror. The hypotenuse face of the roof prism separated a distance from, and tilted with respect to, the flat mirror such that a light beam reflected by the flat mirror will be reflected back to a different point on the flat mirror via the internal reflections of the roof prism. Preferably, at least one refocusing optical element is positioned in the light path of a beam between the flat mirror and the roof prism to restrict the divergence of the light beam(s). An output mirror reflects each light beam emerging from the flat mirror that is not reflected back to the roof prism. At least one receiving device receives the light beam(s) reflected by the output mirror and extracts the delayed signal from the light beam(s).
The present invention also includes another roof prism-containing apparatus for optically generating time delays in signals. An input light source generates at least one individual light beam from at least one direction. An input mirror reflects the individual light beam(s) to a spatial light modulator. The spatial light modulator then reflects a light beam to a roof prism, the hypotenuse face of the roof prism being separated a distance from, and tilted with respect to, the spatial light modulator such that a light beam reflected by the spatial light modulator will be reflected back to a different point on the spatial light modulator via internal reflection in the roof prism. At least one refocusing optical element is used to restrict the divergence of the light beam(s). An output mirror reflects each beam emerging from the spatial light modulator that is not reflected back to the roof prism. At least one receiving device extracts the delayed signal from the light beam(s) reflected by the output mirror and determines the delay in the beam(s).